The prior art will be explained on the basis of e.g. an acoustooptic Bragg cell using an LiNbO.sub.3 waveguide path disclosed in "HIKARI SHUSEKI KAIRO" by H. Nishihara et al., published by OHM Co., page 328, and an SAW light deflecting device disclosed in the lecture of the 35-th OYO BUTSURI GAKU KANKEI RENGO KOENKAI 28a-ZQ-4. FIG. 1 shows the construction of the prior art light deflecting device. In FIG. 1, a Ti diffused waveguide layer 28 is formed on an LiNbO.sub.3 substrate 27. Laser light 30 incident on an input prism 29 becomes waveguide light 31 propagating through the waveguide layer 28. The waveguide light 31 is separated into transmitted waveguide light 33 and diffracted waveguide light 34 by a SAW light deflecting device 32. Both waveguide lights 33 and 34 are radiated from an output prism 35 as transmitted light 36 and diffracted light 37. The diffraction angle of the diffracted waveguide light 34 depends on the pitch of an elastic wave 38 produced by the SAW light deflecting device 32, and the diffraction angle of the diffracted waveguide light 34 and hence the radiation direction of the diffracted light 37 can be changed by an electric signal applied to the SAW light deflecting device 32.
The prior art light deflecting device, however, has the following disadvantages. A change of the diffraction angle caused by the SAW light deflecting device 32 is slight, so that it has been impossible to deflect the diffracted light greatly by the SAW light deflecting device 32. Further, what is deflected is diffracted light, so that the efficiency of the use of energy of the input light 30 has been low.